Abstract

(TMA)2[OsF6] has at room temperature a rhombohedral structure (space group R3̄) which is pseudo-cubic (K2[PtCl6]-type). a=8.0565(6) Å, c = 20.020(3) Å (hexagonal setting). Double refraction decreases linearly with increasing temperature and disappears above 434 K which indicates cubic symmetry of the high temperature phase. The change from the cubic high temperature structure to the rhombohedral structure is characterized by a ferrorotative tilt of the [OsF6] octahedra and the TMA tetrahedra. Evidence of ferroelasticity follows from the switching of the trigonal c-axis by the application of directional pressure. The phase transitions of (TMA)2[OsF6] (Fm3̄m → R3̄) and of the recently reported (TMA)2[PtF6] and (TMA)2[IrF6] (Fm3̄m → I4/m) are described by Landau’s theory.